Exhaust silencer assembly

ABSTRACT

An exhaust silencer assembly for use with a gas turbine engine, the exhaust silencer assembly comprising an exhaust duct extension disposed downstream from an exhaust diffuser of the gas turbine engine, and an exhaust silencer disposed downstream from the exhaust duct extension. The exhaust duct extension has a length that is at least about 50% of an inner diameter of the exhaust duct extension.

BACKGROUND

The present invention relates to noise suppression systems. In particular, the present invention relates to noise suppression systems for use with gas turbine engines such as aircraft auxiliary power unit (APU) turbine engines.

Large commercial aircraft typically include on-board APU turbine engines, located in the tail sections of the aircraft, to provide electrical power and compressed air for systems throughout the aircraft. When an aircraft is on the ground, the primary propulsion engines of the aircraft are shut down, and the APU turbine engine provides the main source of power for a variety of systems, such as the environmental control systems, hydraulic pumps, electrical systems, and main engine starters. The APU turbine engine may also provide power during in-flight operations, such as for electrical and pneumatic systems.

In many gas turbine engine applications, particularly those in which the engine is used in conjunction with a commercial passenger aircraft, there is a widespread demand by the airline industry to maintain noise levels below defined limits. This is particularly important at ground service stations for the aircraft, where ground crew load and unload luggage, fuel and provision the aircraft, and remove waste materials from the aircraft. Under these conditions, the aircraft APU is the turbine engine of interest.

One technique for attenuating the exhaust noise of an APU turbine engine involves placing an exhaust silencer directly downstream from the APU exhaust diffuser. This allows the exhaust silencer to attenuate the noise of the combustion gases as the gases exit the exhaust diffusers. The exhaust silencer is typically placed directly downstream from the APU exhaust diffuser to minimize the overall axial length. However, the combustion gases exit the APU at high velocities (e.g., greater than 500 feet/second), and create a turbulent mixing downstream from the exhaust diffuser. This is particularly true when cooling air from an eductor mixes with the combustion gases. The turbulent mixing generates a substantial amount of additional noise downstream from the exhaust diffuser. Thus, the downstream noise is generated after the combustion gases enter the exhaust silencer, which reduces the effectiveness of the exhaust silencer.

One solution to this issue involves increasing the size of the exhaust silencer to accommodate for the loss in efficiencies. However, exhaust silencers are relatively large (typically equal in size to the APU, or larger) and undesirably increase the overall weight of the aircraft. As such, there is a need for a noise suppression system that is effective for attenuating downstream noise without increasing the size and weight of the aircraft tail section.

SUMMARY

The present invention relates to an exhaust silencer assembly for use with a gas turbine engine, such as an APU turbine engine. The exhaust silencer assembly includes an exhaust duct extension disposed downstream from an exhaust diffuser of the gas turbine engine, and an exhaust silencer disposed downstream from the exhaust duct extension. The exhaust duct extension has a length that is at least about 50% of an inner diameter of the exhaust duct extension, thereby allowing downstream noise to substantially generate prior to reaching the exhaust silencer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top schematic view of an aircraft tail section, which includes an exhaust silencer assembly in use with an on-board APU turbine engine.

FIG. 2 is a top schematic view of an aircraft tail section, which includes an alternative exhaust silencer assembly in use with an on-board APU turbine engine.

FIG. 3 is a graphical representation of insertion loss results versus frequency for an exemplary exhaust silencer assembly of the present invention and a comparative exhaust silencer assembly.

DETAILED DESCRIPTION

FIG. 1 is a top schematic view of aircraft tail section 10, which includes exterior structure 12 and APU nacelle 14. APU nacelle 14 is a compartment within exterior structure 12 that contains APU 16, eductor 18, exhaust silencer assembly 20, and exhaust pipe 22, where exhaust silencer assembly 20 attenuates the exhaust noise of APU 16 during operation. Thus, exhaust silencer assembly 20 is beneficial for use in commercial aircraft to meet aviation noise standards. APU 16 is an on-board APU turbine engine that includes turbine portion 24 and exhaust diffuser 26, and which provides electrical power to the aircraft. APU 16 also includes additional components (not shown) that facilitate the operation of APU 16 and the transfer of electrical power (e.g., inlet air ducts, gearboxes, and generators). Although commonly located in aircraft tail section 10, any suitable location on the aircraft could be utilized.

Eductor 18 is an airflow system that extends annularly around at least a portion of exhaust diffuser 26 and draws cooling air through the compartment and/or an oil cooler (not shown). Exhaust silencer assembly 20 is disposed downstream from eductor 18 and exhaust diffuser 26, and is configured to attenuate the noise of APU 16. Exhaust pipe 22 is disposed downstream from exhaust silencer assembly 20, and provides a channel for expelling the gases from aircraft tail section 10. The term “downstream” is used herein with reference to the direction of the combustion gas flow from APU 16, as represented by directional flow arrow 28 in FIG. 1.

During the course of operation, turbine portion 24 of APU 16 compresses combustion air, adds fuel, and combusts the resulting fuel/air mixture. The resulting hot, high-pressure combustion gas then expands through a turbine stage (not shown) within turbine portion 24. The resulting rotation of the turbines is used to generate electrical power for associated devices of the aircraft (not shown). The spent combustion gases exit through exhaust diffuser 26, and flow into exhaust silencer assembly 20. Cooling air from APU nacelle 14 is drawn in through eductor 18, and mixes with the combustion gases.

Exhaust silencer assembly 20 is a noise suppression system that includes exhaust duct extension 30 and exhaust silencer 32. Exhaust duct extension 30 is an annular metal tube located downstream from exhaust diffuser 26 and eductor 18. As shown, exhaust duct extension 30 includes upstream end 30 a secured to eductor 18 and downstream end 30 b secured to exhaust silencer 30. In alternative embodiments in which eductor 18 is omitted, upstream end 30 a of exhaust duct extension 30 may be secured to exhaust diffuser 26 of APU 16.

Exhaust silencer 32 is a noise attenuating system located downstream from exhaust duct extension 30, and includes first stage 32 a and second stage 32 b. First stage 32 a is located directly downstream from exhaust duct extension 30 and has a large diameter to attenuate low-frequency noise. Second stage 32 b is located directly downstream from first stage 32 a, and has a smaller diameter to attenuate higher-frequency noise. In alternative embodiments, exhaust silencer 32 may include additional numbers of stages to attenuate noise from APU 16 and the combustion and cooling gases.

As discussed above, the turbulent mixing of the combustion gases and the cooling gases generates a substantial amount of downstream noise. If exhaust silencer 32 were located directly downstream from exhaust diffuser 26 and eductor 18, as typically found with current APU arrangements, the downstream noise would generate after the combustion gases reached exhaust silencer 32. This would reduce the effectiveness of exhaust silencer 32. However, the dimensions of exhaust duct extension 30 provide suitable residence times for the combustion gases to generate the downstream noise before reaching exhaust silencer 32. As shown, exhaust duct extension 30 has an inner diameter 30 _(D) orthogonal to directional flow arrow 28, and a length 30 _(L) parallel to directional flow arrow 28, where length 30 _(L) is measured between upstream end 30 a to downstream end 30 b.

Examples of suitable diameters for diameter 30 _(D) range from about 20 centimeters (about 8 inches) to about 66 centimeters (about 26 inches), with particularly suitable diameters ranging from about 25 centimeters (about 10 inches) to about 51 centimeters (about 20 inches). Suitable lengths for length 30 _(L) include at least about 50% of diameter 30 _(D). Lengths for length 30 _(L) that are less than about 50% of diameter 30 _(D) result in residence times that are too short for generating the downstream noise. Accordingly, examples of suitable lengths for length 30 _(L) range from about 50% of diameter 30 _(D) to about 300% of diameter 30 _(D). Lengths for length 30 _(L) greater than about 300% of diameter 30 _(D) do not provide any additional noise attenuation benefits, and undesirably increase the size of exhaust silencer assembly 20. Examples of particularly suitable lengths for length 30 _(L) range from about 100% of diameter 30 _(D) to about 200% of diameter 30 _(D). The ratios of length 30 _(L) to inner diameter 30 _(D) determine the volume within exhaust duct extension 30, which correspondingly affects the turbulent mixing of the combustion and cooling gases. These suitable and particularly suitable diameters and lengths provide the combustion gases a suitable residence time to generate downstream noise before reaching exhaust silencer 32.

In addition to increasing noise attenuation, the use of exhaust duct extension 30 also reduces the weight and overall volume of exhaust silencer assembly 20 compared to standard exhaust silencer assemblies. A standard exhaust silencer assembly typically includes one or more large entrance stages at the location of exhaust duct extension 30 (i.e., between eductor 18 and first stage 32 a). However, the same acoustic performance can be achieved by replacing the large entrance stage(s) with exhaust duct extension 30. This reduces weight and overall volume of aircraft tail section 10, thereby increasing flight efficiencies of the aircraft.

FIG. 2 is a top schematic view of aircraft tail section 34, which is an alternative embodiment to aircraft tail section 10 (shown in FIG. 1). Aircraft tail section 34 includes exterior structure 36 and APU nacelle 38, where APU nacelle 38 contains APU 40, eductor 42, exhaust silencer assembly 44, and exhaust pipe 46. APU 40 is an on-board APU turbine engine that includes turbine portion 48 and exhaust diffuser 50, and which functions in the same manner as APU 16 (shown in FIG. 1) for providing electrical power to the aircraft. The directional gas flow from APU 40 is represented in FIG. 2 by directional flow arrow 52. Similarly, eductor 42 and exhaust pipe 46 function in the same manners as eductor 18 and exhaust pipe 22 (shown in FIG. 1).

Exhaust silencer assembly 44 is a noise suppression system that includes exhaust duct extension 54 and integrated exhaust silencer 56. Exhaust duct extension 54 is an annular metal tube located downstream from exhaust diffuser 50 and eductor 42, and functions in the same manner as exhaust duct extension 30 (shown in FIG. 1). Thus, exhaust duct extension 54 provides a suitable residence time for the combustion gases to generate the downstream noise before reaching integrated exhaust silencer 56. Exhaust duct extension 54 includes upstream end 54 a secured to eductor 42 and downstream end 54 b secured to integrated exhaust silencer 56. In alternative embodiments in which eductor 42 is omitted, upstream end 54 a of exhaust duct extension 54 may be secured to exhaust diffuser 50 of APU 40.

Exhaust duct extension 54 has an inner diameter 54 _(D) orthogonal to directional flow arrow 52, and a length 54 _(L) parallel to directional flow arrow 52, where length 54 _(L) is measured between upstream end 54 a to downstream end 54 b. Suitable and particularly suitable dimensions for length 54 _(L) and inner diameter 54 _(D) include those discussed above for length 30 _(L) and inner diameter 30 _(D) of extrusion duct extension 30. As discussed above, the dimensions of exhaust duct extension 54 provide suitable residence times for the combustion gases to generate the downstream noise before reaching exhaust silencer 56.

Integrated exhaust silencer 56 is a noise attenuating system located downstream from exhaust duct extension 54, and includes a plurality of annular baffles that form resonant chambers 56 a-56 d to attenuate the noise of APU 40 and the combustion and cooling gases. An example of a suitable noise suppression system for exhaust silencer 54 includes the system disclosed in Sheoran et al., U.S. Patent Publication No. 2002/0139120. Exhaust silencer 32 (shown in FIG. 1) and integrated exhaust silencer 56 are examples of suitable exhaust silencers for use with exhaust duct extensions (e.g., exhaust duct extensions 30 and 54) to increase the attenuation of noise from the APUs (e.g., APUs 16 and 40), while also reducing the weight and overall volume of the aircraft tail sections (e.g., aircraft tail sections 10 and 34).

EXAMPLES

The present invention is more particularly described in the following examples that are intended as illustrations only, since numerous modifications and variations within the scope of the present invention will be apparent to those skilled in the art. Acoustic insertion losses of an exemplary exhaust silencer assembly of the present invention (Example 1) and a comparative exhaust silencer assembly (Comparative Example A) were quantitatively measured to compare their noise attenuation capabilities. The acoustic insertion loss measurement initially involved measuring the acoustic levels emitted from an aircraft tail section over a range of frequencies. The aircraft tail section included an APU turbine engine, eductor, and exhaust pipe, but did not include an exhaust silencer assembly. This provided baseline acoustic levels for the exhaust silencer assemblies of Example 1 and Comparative Example A.

The exhaust silencer assembly of Example 1 included the same arrangement as aircraft tail section 34 (shown in FIG. 2), where the exhaust duct extension had an inner diameter of about 41 centimeters (16 inches) and a length of about 61 centimeters (24 inches). As such, the length of the exhaust duct extension was 150% of the inner diameter of the exhaust duct extension. The acoustic levels emitted from the aircraft tail section were then measured over a range of frequencies in the same manner as for the baseline acoustic levels. The acoustic insertion losses for the exhaust silencer assembly of Example 1 (at each measured frequency) were then calculated as the differences between the acoustic levels for Example 1 and the baseline acoustic levels, as represented below by Equation 1. Thus, the acoustic insertion losses measured the amount noise attenuation achieved with the exhaust silencer assembly of Example 1.

AcousticInsertionLoss_(Ex.1)=(AcousticLevels_(Baseline))−(AcousticLevels_(Ex.1))  Equation 1

The exhaust silencer assembly of Comparative Example A included an arrangement similar to aircraft tail section 34, except that the exhaust silencer assembly of Comparative Example A did not include an exhaust extension duct, and the integrated exhaust silencer was located directly downstream from the exhaust diffuser and eductor. The acoustic levels emitted from the aircraft tail section were then measured over a range of frequencies in the same manner as for the baseline acoustic levels and the acoustic levels of Example 1. The acoustic insertion losses for the exhaust silencer assembly of Comparative Example A (at each measured frequency) were then calculated as the differences between the acoustic levels for Comparative Example A and the baseline acoustic levels, as represented below by Equation 2.

AcousticInsertionLoss_(Comp.Ex.A)=(AcousticLevel_(Baseline))−(AcousticLevels_(Comp.Ex.A))  Equation 2

FIG. 3 is a graphical representation of the acoustic insertion loss results versus frequency for the exhaust silencer assemblies of Example 1 and Comparative Example A. As shown, the acoustic insertion losses for the exhaust silencer assembly of Example 1 exceed those of Comparative Example A over most of the measured frequency range. This is believed to be due to the use of the exhaust duct extension, which allows the combustion gases to generate downstream noise before reaching the integrated exhaust silencer. As such, the integrated exhaust silencer is capable of attenuating substantially all of the generated downstream noise. In contrast, for the exhaust silencer assembly of Comparative Example A, the combustion gases reached the integrated exhaust silencer before generating the downstream noise. As a result, the integrated exhaust silencer used for Comparative Example A was not able to attenuate all of the downstream noise, thereby reducing the efficiency of the integrated exhaust silencer. Accordingly, the exhaust silencer assembly of Example 1 provides increased noise attenuation without increasing the overall volume or weight of the aircraft tail section.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. 

1. An exhaust silencer assembly, the exhaust silencer assembly comprising: an exhaust duct extension disposed downstream from an exhaust diffuser, the exhaust duct extension having a length and an inner diameter, wherein the length of the exhaust duct extension is at least about 50% of the inner diameter of the exhaust duct extension; and an exhaust silencer disposed downstream from the exhaust duct extension.
 2. The exhaust silencer assembly of claim 1, wherein the length of the exhaust duct extension ranges from about 50% of the inner diameter of the exhaust duct extension to about 300% of the inner diameter of the exhaust duct extension.
 3. The exhaust silencer assembly of claim 2, wherein the length of the exhaust duct extension ranges from about 100% of the inner diameter of the exhaust duct extension to about 200% of the inner diameter of the exhaust duct extension.
 4. The exhaust silencer assembly of claim 1, wherein the inner diameter of the exhaust duct extension ranges from about 20 centimeters to about 66 centimeters.
 5. The exhaust silencer assembly of claim 4, wherein the inner diameter of the exhaust duct extension ranges from about 25 centimeters to about 51 centimeters.
 6. The exhaust silencer assembly of claim 1, wherein the exhaust duct extension has a first end secured to the exhaust diffuser.
 7. The exhaust silencer assembly of claim 1, wherein the gas turbine engine comprises an auxiliary power unit turbine engine.
 8. The exhaust silencer assembly of claim 1, wherein the exhaust silencer comprises an integrated exhaust silencer.
 9. An exhaust silencer assembly for use with an auxiliary power unit turbine engine, the exhaust silencer assembly comprising: an exhaust duct extension located downstream from the auxiliary power unit turbine engine for receiving combustion gases emitted from the auxiliary power unit turbine engine, the exhaust duct extension having dimensions effective for allowing the combustion gases to generate downstream noise prior to exiting the exhaust duct extension; and an exhaust silencer located downstream from the exhaust duct extension and configured to at least partially attenuate the downstream noise of the combustion gases.
 10. The exhaust silencer assembly of claim 9, wherein the dimensions of the exhaust duct extension comprise a length of the exhaust duct extension and an inner diameter of the exhaust duct extension, the length of the exhaust duct extension being at least about 50% of the inner diameter of the exhaust duct extension.
 11. The exhaust silencer assembly of claim 10, wherein the length of the exhaust duct extension ranges from about 50% of the inner diameter of the exhaust duct extension to about 300% of the inner diameter of the exhaust duct extension.
 12. The exhaust silencer assembly of claim 11, wherein the length of the exhaust duct extension ranges from about 100% of the inner diameter of the exhaust duct extension to about 200% of the inner diameter of the exhaust duct extension.
 13. The exhaust silencer assembly of claim 9, wherein the exhaust silencer comprises an integrated exhaust silencer.
 14. The exhaust silencer assembly of claim 9, wherein the exhaust duct extension and the exhaust silencer are located in an aircraft tail section.
 15. An auxiliary power unit assembly comprising: an auxiliary power unit turbine engine having an exhaust diffuser; an eductor disposed around at least a portion the exhaust diffuser; an exhaust duct extension having a first end secured to the eductor and a second end, the exhaust duct extension having a length between the first end and the second end and a diameter, wherein the length of the exhaust duct extension is at least about 50% of the diameter of the exhaust duct extension; and an exhaust silencer secured to the second end of the exhaust duct extension.
 16. The auxiliary power unit assembly of claim 15, wherein the length of the exhaust duct extension ranges from about 50% of the diameter of the exhaust duct extension to about 300% of the diameter of the exhaust duct extension.
 17. The auxiliary power unit assembly of claim 16, wherein the length of the exhaust duct extension ranges from about 100% of the diameter of the exhaust duct extension to about 200% of the diameter of the exhaust duct extension.
 18. The auxiliary power unit assembly of claim 15, further comprising an exhaust pipe secured to the exhaust silencer at a location downstream from the exhaust silencer.
 19. The auxiliary power unit assembly of claim 15, wherein the exhaust silencer comprises an integrated exhaust silencer.
 20. The exhaust silencer assembly of claim 15, wherein the exhaust duct extension and the exhaust silencer are located in an aircraft tail section. 